If you don't like the blackness of regular graphite, you can find a white paste graphite that works great. Search online for "Rene Martinez GraphitAll Guitar Lube" if you're interested. I don't use it much on my nylons, but on steel strings, it is really nice to have.

Another suggestion is micron size Teflon dust....Labelle Industries 134PTFE is a brand that I like but shipping is prohibitive. Amazon also lists ptfe powder lubricant. It can be "puffed" into the space around the tuning rollers without staining anything. A very small amount on the nut will eventually work its way into the bone and provide long term lubrication. The stuff looks sort of like white flour but is very slick.